Have You Had This COVID Vaccine Side Effect, Too?
From arm soreness to fever, here’s what you should be prepared after your first or second dose.
There has never been a moment in our history when vaccines have gotten this much hype. Thanks to the miracle of modern science, we now have multiple highly effective vaccines to help us end the COVID-19 pandemic.
But with any new scientific breakthrough—especially one of this scale—comes a fair amount of scrutiny. Getting a new vaccine can be scary, even when experts agree that it’s extremely effective and safe. And with the internet being what it is these days, it’s easy to get caught up in anecdotal reports about weird side effects after vaccination.
To cut through the noise, we spoke to two experts about the most common COVID vaccine side effects and why they occur—plus, when it’s worth reaching out to a doctor.
How the Vaccines Work
Before we dive into side effects, let’s talk about the vaccines themselves. In the United States, we have three different COVID vaccines available: Pfizer and Moderna, which are both mRNA vaccines that require two doses, and Johnson & Johnson, an adenovirus vector vaccine that requires just one dose.
Why do we need two doses of the mRNA vaccines? In clinical trials, scientists found that a second dose was needed to strengthen the body’s immune response enough to get to that 90+% efficacy rate. Dose one confers some level of immunity, and dose two acts as the “booster” to offer even more protection.
Similarly, the Johnson & Johnson vaccine was tested as a single dose shot with a 66% efficacy rate. We don’t know what would happen if you got two doses of this vaccine—and in general, it’s best to trust the data borne out in clinical trials.
For the vaccines that require two doses, side effects differ somewhat between the first and second shots. “People who have a second dose may have a stronger response than the first dose, but there is some variability,” says Scott Weisenberg, M.D., an infectious diseases specialist at NYU Langone Health in New York City. This is because of the way your immune system reacts to the vaccine. The first dose introduces the coronavirus spike protein and teaches your body to build an immune response. Once you get the second shot, your immune system immediately recognizes it and jumps in to react. This also explains why people who have had COVID-19 tend to get side effects after their first dose. “People who have had the coronavirus itself may be more likely to have symptoms when they get the vaccine because they are getting more of a booster,” Weisenberg explains. Their immune system recognized the spike protein from its previous experience fighting the virus.
COVID Vaccine Side Effects
Not everyone will have these side effects (many people have none), but here’s what to be prepared for when you get your COVID shot(s):
Local injection-site reactions. The most obvious vaccine side effect, and probably the most common, is soreness or discomfort in the arm where you got the shot. “In part, that’s driven by the vaccine, but it’s also partly driven by the fact that we’re sticking a needle into a muscle, and that can cause some local irritation,” explains Ethan Smith, PharmD, a clinical pharmacist at Cedars-Sinai Medical Center in Los Angeles. “It’s always possible to get some bruising and pain at the injection site.”
In rarer cases, you may notice a rash on your arm several days after vaccination. This is being referred to anecdotally as “COVID arm,” and experts don’t yet know what causes it. It looks like “a swollen, red area on the arm about eight days after initial vaccination,” Dr. Weisenberg says. “We’re discovering this as more and more people are getting the vaccine.” It appears to be harmless and to go away after several days.
Lymph node swelling. In January, the Society of Breast Imaging (SBI) released a report outlining the possibility of lymph node swelling after receiving the COVID vaccine. This occurs in around 10-20% of people and can sometimes show up on imaging tests like mammograms or MRIs. Understandably, it can cause confusion if you’re not expecting it.
Here’s why it happens: Your lymphatic system houses most of the immune cells in your body. “When we get vaccinated, we have an immune response that is local, meaning these immune cells have to come to the site of vaccination to respond to this antigen or vaccine,” Smith explains. The closest lymph nodes to the injection site are located near your armpits, so that’s where the swelling is most likely to occur. It usually resides within seven to 10 days.
Systemic fatigue and/or fever. You probably know at least one person who has gotten fever symptoms after their first or second COVID shot. While uncomfortable, this is nothing to worry about; it’s a sign that your body is responding appropriately to the vaccine. “You’re noticing your body’s response to the vaccine as your immune system starts kicking in,” Dr. Weisenberg says.
To put it another way, you may feel fatigued because your body is putting energy toward building an immune response. “That’s your body’s response to being in a state of immune reaction,” Smith says. “It’s to preserve resources for your immune system. You feel tired because you don’t want to expend energy on other things [so your] immune system [can] function properly.”
It appears—though this isn’t true across the board—that younger people are the most likely to get these systemic side effects. “As you age, your immune system becomes less active,” Smith says, so older adults may not notice as strong an immune response. Don’t fret, though; the vaccine will still work regardless of your age! “Everybody is different,” Dr. Weisenberg affirms. “What we know is that the vaccine is highly effective in all age groups. There are also plenty of people who do not get side effects, and they still do very well with the vaccine.”
Menstrual cycle changes. Some women have been reporting changes to their menstrual cycle after receiving the COVID vaccine. This topic has gotten a lot of buzz online, but the limited research around it is inconclusive and experts still need to sort out whether menstrual chances are coincidental or indeed directly related to vaccination. “Many side effects that are not as common may not show up in a clinical trial," Dr. Weisenberg says. "So they’ll be looked at very carefully during this next phase.”
If there is a connection, Smith says, it may have to do with the immune system’s role in menstruation. “We typically think of the menstrual cycle as being primarily hormone-driven,” he notes. “But there is also a component of the immune system involved. In any illness or vaccine that is activating the immune system, you may see some minor or mild [menstruation] changes—either differences in timing, or heavier or lighter periods than normal.”
Allergic reactions. With any vaccine or medication, there is the rare chance of severe allergic reactions, especially in people with a history of related allergies. The COVID vaccines are no exception. The CDC has reported 21 cases of anaphylaxis after receipt of the Pfizer vaccine (that was out of 2 million doses administered) and 10 cases after receipt of the Moderna vaccine (out of 4 million doses). Most of these patients had a prior history of allergies. “It’s fairly well-known that people can have a syndrome called multiple allergy syndrome, whereby if they are allergic to one thing, they may have a higher propensity to be allergic to others,” Smith explains. All patients ultimately recovered.
Medical professionals are well-trained to manage these kinds of events, so rest assured that you are in good hands. Most vaccine providers require people to wait at least 15 minutes—maybe more, if they have an allergic history—after getting the shot before they can leave the clinic. And for those people who do have an allergic reaction to their first shot, the CDC recommends they not get a second dose.
Blood clots. We’d be remiss not to talk about blood clots, but before we do, keep in mind that they are exceedingly rare. On April 13, 2021, the CDC implemented a pause in distribution of the Johnson & Johnson vaccine after six reported events of blood clotting in the weeks following vaccination—a precaution the agency reversed on April 23 after determining the vaccine is safe. “We’re talking less than one in one million reactions for these types of clotting events,” Smith says. Research has shown that you’re at least 10 times more likely to develop blood clots from COVID-19 itself than from the vaccine.
Blood clots have also been reported in several European countries following the AstraZeneca vaccine. While the mechanism isn’t totally clear, scientists have a guess as to why this can happen. “When you get vaccinated, and this immune activation is occurring, the body generates antibodies against the virus,” Smith says. “Very rarely, we can see some stray antibodies develop. … These stray antibodies can attack the platelets, which are components of your blood that help you form clots.” This causes clumps of antibodies and platelets to form, which may lead to a blood clot.
Smith notes that this type of clotting can sometimes occur with other medical treatments. “This is not something that is potentially unique to COVID vaccines,” he says. “There are other medications—one is a common anti-coagulant or blood thinner that we use in the hospital—that in rare instances are also associated with the same phenomenon.” It also helps to put this risk level into context. “The odds of being struck by lightning in your lifetime are approximately one in 15,300,” he says. “So, you have a better chance of being struck by lightning than you do of having a blood clot.”
Still, the Johnson & Johnson pause shows that the public health system is (rightfully) paying attention to people’s responses to these vaccines. “The important thing is making sure that we are talking about and studying these reactions,” Smith says, “so on the rare chance they do occur, we know how to intervene appropriately.”
When to Call Your Doctor
How would you know when your vaccine side effects might require medical attention? “If you have symptoms that are persisting or would otherwise be worrisome, let your doctor know,” Weisenberg says. You may be dealing with an unrelated health issue that coincidentally occurred after vaccination. “You want to be careful and not ascribe [all symptoms] to the vaccine that aren’t in the normal realm of things.”
Use these two measures to decide whether you need to see a physician:
Severity. Side effects to COVID vaccines should be mild to moderate and shouldn’t severely disrupt your life. You might have to take a sick day from work, but if your fever is so high the thermometer isn’t registering, or if you’re having the worst headache of your entire life, these are things to be concerned about.
Time. “For these general, systemic side effects, if they don’t resolve in two to three days or you are fatigued for more than a week, that’s something worth calling a physician about,” Smith says. Lymph node swelling and “COVID arm” are the two exceptions to this, as they may last for a week or two after the shot.
Everyone’s reaction to the COVID vaccines will be slightly different—some people won’t feel any adverse effects, while others will feel under the weather for a day or two. What we know for sure is that these vaccines are exceedingly safe and effective.
“The most important thing I want to stress is that these vaccines are extremely safe,” Smith says. “We have a lot of attention on them right now because of the nature of the pandemic and millions of people being vaccinated.” But the truth is that millions of Americans are being vaccinated every day without issue, and we are finally on our way to resuming normal life again. “These vaccines are really the key to getting our lives and the world back on track to where we were pre-pandemic,” Smith says. And that’s the best news we’ve heard in a long time.
- COVID Arm: New England Journal of Medicine. (2021.) “Delayed Large Local Reactions to mRNA Vaccines.” nejm.org/doi/full/10.1056/NEJMc2104751
- Lymph Node Swelling: Society of Breast Imaging. (2021.) “SBI Recommendations for the Management of Axillary Adenopathy in Patients with Recent COVID-19 Vaccination.” sbi-online.org/Portals/0/Position%20Statements/2021/SBI-recommendations-for-managing-axillary-adenopathy-post-COVID-vaccination.pdf
- Pfizer Allergic Reactions: Centers for Disease Control and Prevention. (2021.) “Allergic Reactions Including Anaphylaxis After Receipt of the First Dose of Pfizer-BioNTech COVID-19 Vaccine—United States, December 14-23, 2020.” cdc.gov/mmwr/volumes/70/wr/mm7002e1.htm
- Moderna Allergic Reactions: Centers for Disease Control and Prevention. (2021.) “Allergic Reactions Including Anaphylaxis After Receipt of the First Dose of Moderna COVID-19 Vaccine—United States, December 21,2020-January 10, 2021.” cdc.gov/mmwr/volumes/70/wr/mm7004e1.htm
- J&J Distribution Pause: Centers for Disease Control and Prevention. (2021.) “Joint CDC and FDA Statement on Johnson & Johnson COVID-19 Vaccine.” cdc.gov/media/releases/2021/s0413-JJ-vaccine.html
- COVID Blood Clot Risk: BMJ. (2021.) “Covid-19: Risk of cerebral blood clots from disease is 10 times that from vaccination, study finds.” bmj.com/content/373/bmj.n1005